A tendency for information multi-media has developed and the amount of information has increased rapidly. Optical recording media such as CDs have been often used for the media to record such high-volume data. However, the reliability of the information reproduction is not certain from many points of view. Various efforts are taken to improve accuracy and reliability in reading the information.
FIG. 17 shows a structure of a conventional optical pick-up head unit using a CD as an optical recording medium. In FIG. 17, 1 denotes a light source, 7 denotes a divergent beam, 23 denotes an object lens, 41 denotes an optical recording medium, 51 denotes a photodetector, 60 denotes a diffraction grating, 61 denotes a half mirror, and 91 and 92 denote actuators respectively.
In FIG. 17, the light source 1 is a semiconductor laser that radiates beams 780 nm in wavelength. The divergent beam 7 radiated from the light source 1 enters the diffraction grating 60 and becomes three beams 7A, 7B, and 7C. The beam 7A is a zero-order diffracted light while 7B and 7C denote .+-.first-order diffracted light. For simplifying, those three beams (7A, 7B, 7C) are shown by only one beam in FIG. 17.
The three beams (7A, 7B, 7C) generated at the diffraction grating 60 enter the half mirror 61, and half of the light quantity is reflected to head for the object lens 23. The object lens 23 is a finite system object lens to focus the beams (7A, 7B, 7C) on the optical recording medium 41. The numerical aperture of the object lens 23 is 0.08 at the light source 1 side and 0.45 at the optical recording medium 41 side.
A series of pits about 0.83-3 .mu.m in length (corresponding to 3T-11T that are 8-16 modulated (EFM)) is recorded on an information recording surface 41A located on the optical recording medium 41, at a track pitch of 1.6 .mu.m. Here, T indicates a channel bit cycle. The substrate 41B of the optical recording medium 41 has a thickness of 1.2 mm.
The beams (7A, 7B, 7C) reflected and diffracted at the optical recording medium 41 re-transmit through the object lens 23 before entering the half mirror 61. Half of the quantity of the light beams (7A, 7B, 7C) entering the half mirror 61 transmits through the half mirror 61 to be applied with astigmatism, and are received at the photodetector 51.
Subsequently, a focus error (FE) signal, a tracking error (TE) signal, and a radio frequency (RF) signal that reproduces information recorded on the optical recording medium 41 are generated by using signals outputted from the photodetector 51. The FE signal is detected by astigmatic method, while the TE signal is detected by a three-beam method. The FE signal and the TE signal are supplied to the actuators 91 and 92 for controlling both focus and tracking.
FIG. 18 is a schematic view to show the structure of the photodetector 51. The photodetector 51 is composed of a silicon substrate and includes six receiving parts (511-516) and six I-V converting parts (511A-516A). The beam 7A is received at the receiving parts 511-514, while the beam 7B is received at the receiving part 515 and 7C at the receiving part 516 respectively. The receiving parts 511-516 output current signals corresponding to the quantity of the received light. The I-V converting parts 511A-516A receive the current signals outputted from the receiving parts 511-516, and convert the signals respectively into low-impedance voltage signals. Since signals outputted from the photodetector 51 are converted into voltage signals at the I-V converting parts, they are less influenced by extraneous electric noise.
Musical signals recorded on a CD can be reproduced by rotating the optical recording medium at a linear velocity of 1.2 m/s, and the repeating frequency including marks and spaces in 3T is 720 kHz. The electric circuit requires a band up to about 1.4 MHz at a frequency of -3 dB. The velocity is defined as a single speed. Recently however, CD-ROMs as recording media for recording computer data have become popular to be used as peripheral devices, and information reproducing apparatuses for reproducing data at a high speed of from 30.times. to 40.times. have been developed actively for transferring data as quick as possible. Since the signal band is widened at a higher speed, the signals are more subjected to influences of extraneous noise. Due to the influences of the extraneous noise, data recorded on a medium cannot be reproduced with high reliability even when voltage signals are outputted from the photodetector 51. Especially a CD-RW as a rewritable medium is subjected to influences of the extraneous noise because the medium has a low reflectivity.